This invention generally relates to devices for forming pellets, and is specifically concerned with a punch that compresses granular uranium dioxide into fuel pellets for use in the fuel rods of nuclear fuel assemblies.
Devices for compressing granular materials into pellets are well known in the prior art. Such devices are used to compress granular uranium dioxide into pellets which are stacked into the fuel rods used in the nuclear fuel assemblies that generate heat for converting water into pressurized steam in the core of a nuclear reactor. Such pellet-forming devices typically comprise a punch that is reciprocably movable within an opening in a die. In the devices used to form uranium dioxide fuel pellets, a pair of opposing punches are reciprocably movable throughout opposing openings in a die. In operation, a quantity of granular uranium dioxide is poured into the opening in the die, and, the two opposing punches then compress the powder on either end to form a cylindrical pellet. The pellets are then ejected from the die for further processing.
Within the last few years, the Commercial Nuclear Fuel Division of the Westinghouse Electric Corporation modified its pellet-forming devices to create a chamfer around the edges of the resulting pellets. Such chamfering (which gives the ends of the cylindrical pellets a frusto-conical edge) advantageously reduces the amount of chipping or cracking which may occur at the upper and lower ends of the pellets when the fuel rods which contain them are flexed or otherwise bent by, for example, cross currents in the water that surrounds them during operation. However, in order to achieve such pellet-chamfering, the edges of the substantially flat-faced punches which form the pellet ends had to be chamfered in a complementary manner, which had the effect of providing a sharp, raised edge around each punch. Unfortunately, the provision of such a chamfered edge around the circular face of the punches created an area of localized stress when the punch was momentarily pressed against a trapped quantity of granular uranium dioxide in order to form a hard and substantially non-porous pellet. The applicant has found that such localized stresses around the outer edges of the punches can cause these punches to chip in these areas, which in turn necessitates replacement of the punch. This is a costly shortcoming of such punches, as each punch is a precision tool of significant expense, and further, as each replacement operation holds up production of uranium dioxide fuel pellets.
In an attempt to eliminate or at least reduce the amount of chipping that occurred around the chamfered edges of such punches, the applicant increased the punch toughness by first increasing the particle size of the tungsten carbide particles used in the alloy. This, however, did not have the intended results. Accordingly, a further attempt was made wherein the diameter of the tungsten carbide particles was reduced to less than one micron, and the cobalt content was increased from 8% to approximately 15%. While these changes did significantly increase the toughness of the punch and reduce chipping around the chamfered edges of the punches, it did not completely eliminate such chipping.
Clearly, what is needed is an improved punch having a chamfered edge for use in a pellet forming device which is completely immune from chipping around its chamfered edges. Ideally, such a punch should be fairly easy to fabricate, and further characterized by long life and reliable operation.